Covalent organic frameworks (COFs) are a new class of porous materials that follow the same laws of reticular chemistry like Metal-Organic Frameworks (MOFs). They can be synthesized under relatively mild conditions, using reversible condensation reactions like boronic acid trimerisation, boronate ester formation, trimerization of nitriles and Schiff base reaction. The reversibility of the reactions allows the structural units to self-construct until they achieve the long range periodicity, which results in crystallization of COFs. These frameworks exhibit exceptional high surface areas of up to 3000 m2 g−1 and uniform pore size distributions, and hence considered as promising materials for the storage of gases, separation of gas mixtures, catalysis and charge-carrier transport.
In general, two classes of porous COFs are reported; a) chemically stable porous polymeric structures, often called as PAFs/POPs/CTFs etc. with intrinsic porosity but no crystallinity at all, and b) porous COFs with high crystallinity but moderate or poor chemical stability. COFs derived from B—O, C═N bond formation reactions exhibit low chemical stability due to reversible back reactions which leads to decomposition upon exposure to water vapour and limits their effective use in gas storage, (especially CO2) under practical conditions.
Researchers have attempted to rectify these limitations by alkylation of COF pore walls or by pyridine doping. However, these modifications always lead to decrease in the gas adsorption properties even though it enhances the hydrolytic stability to a moderate extent. A microporous hydrogen-bonded organic framework having high stability and selective adsorption of gas and liquid is reopretd by Xu-Zhong Luo in J. Am. Chem. Soc., 2013, 135 (32), pp 11684-11687 Jul. 25, 2013.
The porphyrin based COFs are good conducting COFs with better charge mobility useful in photo or optoelectronic system is reported in the prior art. Article titled, “Covalent Organic Frameworks with High Charge Carrier Mobility” by Wan, S.; in Chem. Mater. 2011, 23, 4094 reported two covalent organic frameworks (COFs) with structures based on covalently linked porphyrin units and their synthesis.
The synthesis comprises condensation reactions between tetra (p-amino-phenyl) porphyrin TAPP and Terephthaldehyde to obtain (a) COF-366, and condensation reactions between TBPP, and THAn to produce (b) COF-66.
Further the said two porphyrin COFs (COF-366 and COF-66) are determined to be hole conducting with mobilities as high as 8.1 and 3.0 cm2V−1 s−1. Therefore, these multifunctional conducting COFs combine thermal stability, electrical conductivity, high charge mobility, and pore accessibility, which are suitable to design viable plastic electronics and optoelectronic systems.
Omar M. Yaghi in FY 2010 Annual Progress Report discloses synthesis of porphyrin containing COF (UCLA) by imine condensation of tetra(4-aminophenyl)porphyrin with terephthaldehyde to obtain a new porphyrin COF (termed COF-366). Particularly the process for preparation of COF-366 comprises reaction of tetra (4-aminophenyl) porphyrin and terephthalaldehyde in a solvent mixture of ethanol/mesitylene/acetic acid were placed in a pyrex tube. The tube was sealed at 77 K and under vacuum, and heated at 120° C. for three days. The obtained purple powder was washed with absolute ethanol and immersed in anhydrous tetrahydrofuran for 24 h. The solvent was removed under vacuum at room temperature, yielding a porous COF material (yield: 79% based on the porphyrin).
Sharath Kandambeth et al. in J. Am. Chem. Soc., Nov. 15, 2012, 134 (48), pp 19524-19527, discloses synthesis of two chemically stable [acid and water] 2D crystalline covalent organic frameworks (COFs) (TpPa-1 and TpPa-2) using combined reversible and irreversible organic reactions. The said syntheses of these COFs were done by the Schiff base reactions of 1,3,5-triformylphloroglucinol (Tp) with p-phenylenediamine (Pa-1) and 2,5-dimethyl-p-phenylenediamine (Pa-2), respectively, in 1:1 mesitylene/dioxane. Further TpPa-1 and TpPa-2 showed strong resistance toward acid (9N HCl) and boiling water. Moreover, TpPa-2 showed exceptional stability in base (9N NaOH) as well.
Further Xiao Feng in Angewandte Chemie International Edition Volume 51, Issue 11, pages 2618-2622, Mar. 12, 2012 describes a two-dimensional porphyrin covalent organic framework which allows high-rate carrier transport through the porphyrin columns. Also the Synthesis of a phthalocyanine and porphyrin 2D covalent organic framework is reported by Venkata S. Pavan K. Neti et al. in Cryst Eng Comm, 2013, 15, 6892-6895, 31 May 2013.
Since 2D porphyrin-containing COFs have been reported to show high-rate charge carrier conduction and photoconductivity because of the long-range p-orbital overlapping of porphyrin units, the inventors therefore use this keto-enamine COF formation reaction strategy to synthesize chemically stable and crystalline porphyrin-containing COFs. However, this keto-enamine COF formation strategy to synthesize porphyrin-containing COFs may result in the formation of a 3D architecture. Moreover since the proton tautomerism step is an irreversible phenomenon, the chance of the increment of amorphous content in this 3D porphyrin-based COF is much higher. As a result, there may be much less π-π stacking in this amorphous 3D framework, compared to the crystalline 2D porphyrin-containing COFs.
Hence, in order to enhance the chemical stability and crystallinity in 2D porphyrin COFs, inventors have decided to switch to a new strategy to protect the COF interior by introducing —OH functionalities adjacent to the Schiff base [—C═N] centers in COFs and thereby creating an intramolecular O—H.N═C hydrogen bond. This strategy is used to improve the crystallinity, porosity, and chemical stability of the material. Since porphyrin containing COFs have been reported to show high-rate charge carrier conduction and photo conductivity, the synthesis of chemically stable and crystalline porphyrin containing COFs is the need.